MIPS: mm: Add check for highmem before removing memory block

If a device has less physical memory than the highmem threshold
bootmem_init() doesn't set highstart_pfn. This results in highmem_init()
wrongly disabling the entire memory range if the cpu doesn't support
highmem. Add a check that highstart_pfn is non zero before removing the
highmem block.

Fixes: f171b55 ("mips: fix HIGHMEM initialization")
Signed-off-by: Kyle Hendry <kylehendrydev@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Author: kylehendrydev

arch/mips/mm/init.c

@@ -426,10 +426,11 @@ static inline void __init highmem_init(void)
 	unsigned long tmp;
 
 	/*
-	 * If CPU cannot support HIGHMEM discard the memory above highstart_pfn
+	 * If CPU cannot support HIGHMEM discard any memory above highstart_pfn
 	 */
 	if (cpu_has_dc_aliases) {
-		memblock_remove(PFN_PHYS(highstart_pfn), -1);
+		if (highstart_pfn)
+			memblock_remove(PFN_PHYS(highstart_pfn), -1);
 		return;
 	}